1. Technical Field of the Invention
The present invention relates in general to wireless communication systems, and in particular to blind detection of modulation schemes in wireless data communications.
2. Description of Related Art
In connection with the development of third generation mobile communication systems, new wireless multimedia and data applications are being designed and introduced. To support these new applications, improved data transmission technologies are also being developed. One such technology is Enhanced Data rates for Global Evolution (EDGE), which uses a more efficient air-modulation technology that is optimized for data communications and that can be implemented on existing GSM and IS-136 systems. When used in connection with General Packet Radio Service (GPRS), a packet-switched technology that delivers speeds of up to 115 kilobits per second (kbit/s), EDGE technology can increase end user data rates up to 384 kbit/s, and potentially higher in high quality radio environments.
In accordance with EDGE technology, data is transferred in packets via a wireless communication link. A sequence of four consecutive bursts is used for transferring one or two blocks of data. The EDGE system operates according to Link Quality Control (LQC) methods that estimate the predicted channel conditions and, as a result of the estimates, switch a radio transmitter that is used for the data transmissions between robust and less robust modulation schemes. According to EDGE specifications, the four consecutive bursts are transmitted using the same modulation. The four burst sequence can be modulated, for example, using either gaussian minimum shift keying (GMSK) or linear 8-phase shift keying (8-PSK), depending on channel quality predictions. To improve the bit rate to achieve high quality performance, the modulation scheme for the wireless communication link has to change very quickly, without being signaled in advance. As a result, a receiver to which the data packets are being transmitted utilizes blind detection of modulation. In other words, the receiver detects the type of modulation used for each burst based on an analysis of the burst pattern itself.
To accurately receive data, it is important that the receiver detect the type of modulation correctly. If one burst""s modulation scheme is incorrectly detected, one quarter of the bits in the block will be corrupted, thereby preventing accurate data reception. In fact, incorrect detection is actually worse than if one burst is completely lost because the corrupted symbols will have xe2x80x9csoft values,xe2x80x9d which relate to the probability that the bit or bits encoded in each symbol have certain values, that are different than zero. Each soft value will thus favor a bit decision of either a xe2x80x9czeroxe2x80x9d or a xe2x80x9cone.xe2x80x9d The soft values, however, are only valid under the assumption that the correct modulation type was detected during the blind detection procedure. Moreover, depending on the modulation type, each symbol represents a different number of bits. In 8-PSK modulation, for instance, each symbol represents three bits. If the modulation type is not correctly detected, the number of bits assumed to be encoded in each symbol will also be erroneous. In addition, if the Uplink State Flags in the bursts are not correctly decoded by a mobile station that is receiving the transmission, the mobile station might start a transmission at the wrong time, thereby causing interference in the system.
One possible solution for improving the modulation detection decision process is to perform a xe2x80x9cbrute forcexe2x80x9d search, in which the received signals are demodulated according to all of the possible modulation schemes (e.g., through parallel demodulation) and the resulting demodulated signals are analyzed to identify the result that is most likely correct. The problem with this solution is that it requires large amounts of memory and/or additional processing power, which means that this solution is expensive and places an excessive drain on limited resources.
There is a need, therefore, for a method and system for improving receiver performance in connection with burst-wise modulation detection. Such a method and system would preferably improve the accuracy and reliability of blind detection of modulation without requiring large amounts of memory and/or processing resources. In addition, such a method and system would preferably avoid a situation in which an incorrect detection of the modulation scheme is actually worse than a loss of a burst. Furthermore, such a method and system would preferably improve the reliability of Uplink State Flag detections.
The present invention comprises a method and system for blind detection of modulation in a wireless telecommunication network. In accordance with the method, a first modulation scheme is selected for use in transmitting a plurality of bursts. Preferably, the first modulation scheme is selected according to a current channel quality of a radio link between a transmitting station and a receiving station. The plurality of bursts are modulated using the selected first modulation scheme and are received at the receiving station.
After each of the plurality of bursts is received, modulation detection statistics are calculated for each burst. Preferably, the modulation detection statistics include a correlation quality measure between a known training sequence and a training sequence contained within each received burst. Using these modulation detection statistics, a probable modulation scheme that was likely used in transmitting the burst is identified. Once a probable modulation scheme for each of the plurality of received bursts is identified, the various probable modulation schemes are compared to determine if they all match. If not, then the modulation detection statistics are analyzed to identify a single modulation scheme that was most likely used in modulating all of the plurality of bursts.
In accordance with one embodiment of the invention, the modulation detection statistics are analyzed to determine if a majority of the identified probable modulation schemes match. For example, it is determined whether the same modulation scheme has been detected for three out of four bursts. If so, then a xe2x80x9cmajority votexe2x80x9d is taken and the modulation scheme detected for a majority of the received bursts is selected as the single modulation scheme most likely used in modulating all of the bursts.
In some cases, however, an examination of the identified probable modulation schemes will not reveal a majority. For example, if four bursts are transmitted and received, the identified likely modulation scheme for two of the received bursts might differ from the identified probable modulation scheme for the other two bursts. In accordance with another embodiment of the invention, therefore, additional modulation detection information is analyzed to identify the single modulation scheme most likely used in modulating all of the bursts. This additional information can include correlation quality measurements between a known training sequence and the training sequence contained within each of the bursts. Alternatively, this additional information can include historical modulation scheme information or, as an additional alternative, data indicating a bias toward a particular modulation scheme.
In accordance with yet another embodiment of the invention, any faulty bursts for which the probable modulation scheme differs from the identified single modulation scheme are processed to reduce the effect of the faulty bursts. In particular, soft values, which relate to a probability that each symbol in the received bursts has a particular value, are altered for any of the received bursts for which the initially identified probable modulation scheme differs from the identified single modulation scheme. Preferably, such soft values are altered by setting them to a predefined value indicating that each symbol value has equal probability, thus preventing the faulty burst from having an adverse impact on proper decoding of the demodulated bursts. For instance, the predefined value can be zero.
In accordance with the system of the invention, a wireless data communication system includes a receiving station for receiving a plurality of burst signals that are transmitted over an air interface. The receiving station is capable of blind detection of modulation schemes that are used to transmit the plurality of burst signals. The receiving station includes at least one channel estimation device, a modulation detection unit, and a modulation correction unit. The at least one channel estimation device is used for calculating modulation detection statistics for each of the plurality of received burst signals.
The modulation detection unit analyzes these modulation detection statistics for each received burst signal to identify a probable modulation scheme for that received burst signal. Once a probable modulation scheme has been identified for each of the plurality of burst signals, the modulation correction unit compares the various probable modulation schemes to determine whether they all match. If not, the modulation correction unit selects a single modulation scheme most likely used in modulating the plurality of burst signals. This selection can be made using a majority vote procedure for the identified probable modulation schemes or by examining additional modulation detection information to determine a modulation scheme that was most likely used for all of the plurality of burst signals.